1. Field of the Invention
The present invention relates to a novel process for producing open-celled microporous films from high density polyethylene homopolymer resins by mechanical means, and to the films produced thereby.
Porous or cellular films can be classified into two general types: one type in which the pores are not interconnected, i.e., a closed-cell film, and the other type in which the pores are essentially interconnected through tortuous paths which may extend from one exterior surface or surface region to another, i.e., an open-celled film. The porous films of the present invention are of the latter type.
The microporous films of the present invention are characterized by having high film permeabilities. Film "permeability" is a measure of the ease with which a fluid can pass from one exterior surface of a microporous film to another exterior surface. The number of pores, the size of the pores, the degree of pore interconnection and the thickness of a microporous film are among the factors upon which the present invention operates and that influence the permeability of the film. Such permeabilities can be expressed in terms of Gurley seconds, which is the time required for 10 cm..sup.3 of air to pass through 1 in..sup.2 of film from one exterior surface to the other when a pressure differential of 12.2 in. of water is applied across the film. Since permeability is a measure of the ease of mass transfer across the film, lower Gurley second values correspond to lower mass transfer times and hence to higher permeabilities and a greater ease of mass transfer.
2. Background of the Invention
Heretofore, films have been prepared from synthetic resins or polymers, e.g., polypropylene and polyethylene, by various melt extrusion or casting methods. Such films have many desirable properties such as high strength and resistance to heat, light, and various chemicals.
For specific applications such as filter media and battery separators, films having a porous structure in addition to their other properties are necessary or highly desirable.
Porous films have been produced which possess a microporous, open-celled structure. Such films are described, for example, in U.S. Pat. No. 3,426,754, which patent is assigned to the assignee of the present invention. The preferred method of preparation described therein involves drawing or stretching at ambient temperatures, i.e., "cold drawing", a crystalline, elastic starting film in an amount of about 10 to 300 percent of its original length, with subsequent stabilization by heat setting of the drawn film under tension such that the film is not free to shrink or can shrink only to a limited extent. Other processes for producing microporous films involve both "cold" stretching and "hot" stretching steps. For example, U.S. Pat. No. 3,843,761, which patent is also assigned to the assignee of the present invention, describes a process involving annealing, cold stretching, and subsequent multiple hot stretching steps for rendering various polymeric films microporous. In another method, U.S. Pat. No. 4,138,459, for example, also assigned to the assignee of the present invention, describes rendering various polymeric films microporous by a process involving annealing, cold stretching, hot stretching, and heat relaxing steps.
Heretofore there has been no recognition in the art of the factors and variables that affect and effect the permeabilities of the microporous films produced, and, in particular, there has been no such recognition with respect to polyethylene microporous films.